宇宙 (yǔ zhòu) means 'universe' — the totality of all space, time, matter, and energy. The character 宇 (yǔ) originally meant 'roof' or 'space under a roof', and by extension came to signify 'space' or 'the physical world'. 宙 (zhòu) originally referred to 'time' or 'the passage of time', especially in classical texts. Together, 宇宙 conveys the inseparable unity of space and time — the vast, boundless expanse in which everything exists.

This word is used in both scientific and philosophical contexts. It appears frequently in astronomy, cosmology, science education, and also in poetic or reflective speech about humanity’s place in existence. Unlike more colloquial terms like 世界 (shìjiè, 'world'), 宇宙 carries a sense of scale, mystery, and cosmic scope — it emphasizes immensity and fundamental reality rather than human society or geography.